Dwarf planet

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The five IAU-recognized dwarf planets
Ceres - RC3 - Haulani Crater (22381131691).jpg
Ceres (1801)
Pluto in True Color - High-Res.jpg
Pluto (1930)
Eris and dysnomia2.jpg
Eris (2005)
Makemake with moon.JPG
Makemake (2005)
Haumea Hubble.png
Haumea (2004)
The five dwarf planets recognized by the IAU:

A dwarf planet is a planetary-mass object that is neither a true planet nor a natural satellite. That is, it is in direct orbit of a star, and is massive enough for its gravity to compress it into a hydrostatically equilibrious shape (usually a spheroid), but has not cleared the neighborhood of other material around its orbit. [1]

Planet Class of astronomical body directly orbiting a star or stellar remnant

A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.

Natural satellite astronomical body that orbits a planet

A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet.

In fluid mechanics, a fluid is said to be in hydrostatic equilibrium or hydrostatic balance when it is at rest, or when the flow velocity at each point is constant over time. This occurs when external forces such as gravity are balanced by a pressure-gradient force. For instance, the pressure-gradient force prevents gravity from collapsing Earth's atmosphere into a thin, dense shell, whereas gravity prevents the pressure gradient force from diffusing the atmosphere into space.


The term dwarf planet was adopted in 2006 as part of a three-way categorization of bodies orbiting the Sun, [1] brought about by an increase in discoveries of objects farther away from the Sun than Neptune that rivaled Pluto in size, and finally precipitated by the discovery of an even more massive object, Eris. [2] The exclusion of dwarf planets from the roster of planets by the IAU has been both praised and criticized. [3] [4] [5] [6] [7] [8]

IAU definition of <i>planet</i> definition of a planet as a body orbiting the Sun, in hydrostatic equilibrium, having cleared the neighborhood around its orbit; ratified by the IAU in 2006, thereby reclassifying Pluto as a dwarf planet instead

The International Astronomical Union (IAU) defined in August 2006 that, in the Solar System, a planet is a celestial body which:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium, and
  3. has "cleared the neighborhood" around its orbit.
Trans-Neptunian object any object in the Solar System that orbits the Sun at a greater average distance than Neptune

A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has a semi-major axis of 30.1 astronomical units (AU).

Pluto A dwarf planet in the Kuiper belt of the Solar System

Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered and is the largest known plutoid.

As of July 2008 the International Astronomical Union (IAU) recognizes five dwarf planets: Ceres in the asteroid belt, and Pluto, Haumea, Makemake, and Eris in the outer Solar System. [9]

International Astronomical Union Association of professional astronomers

The International Astronomical Union is an international association of professional astronomers, at the PhD level and beyond, active in professional research and education in astronomy. Among other activities, it acts as the internationally recognized authority for assigning designations and names to celestial bodies and any surface features on them.

Ceres (dwarf planet) Dwarf planet and largest asteroid of the main asteroid belt

Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter, slightly closer to Mars's orbit. With a diameter of 945 km (587 mi), Ceres is both the largest of the asteroids and the only unambiguous dwarf planet inside Neptune's orbit. It is the 33rd-largest known body in the Solar System.

Haumea dwarf planet in the Solar System

Haumea is a dwarf planet located beyond Neptune's orbit. It was discovered in 2004 by a team headed by Mike Brown of Caltech at the Palomar Observatory in the United States and independently in 2005, by a team headed by José Luis Ortiz Moreno at the Sierra Nevada Observatory in Spain, though the latter claim has been contested. On September 17, 2008, it was recognized as a dwarf planet by the International Astronomical Union (IAU) and named after Haumea, the Hawaiian goddess of childbirth, though subsequent observations cast doubt on its shape being consistent with hydrostatic equilibrium.

Only two of these bodies, Ceres and Pluto, have been observed in enough detail to demonstrate that they actually fit the IAU's definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto. They subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 (and hence a diameter of ≥838 km assuming a geometric albedo of ≤1) [10] are to be named under the assumption that they are dwarf planets. [11] At the time (and still as of 2019), the only additional bodies to meet this secondary criterion were Haumea and Makemake. However, doubts have since been raised about Haumea. [12]

In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source to that of an idealized flat, fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section.

History of the concept

Pluto and its moon Charon Pluto-Charon-v2-10-1-15.jpg
Pluto and its moon Charon
4 Vesta, one of the largest asteroids Vesta in natural color (cropped).jpg
4 Vesta, one of the largest asteroids

Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the word asteroid for the smaller bodies and then stopped naming or classifying them as planets. [13]

Mars Fourth planet from the Sun in the Solar System

Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war, and is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance that is distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

Asteroid Minor planet that is not a comet

Asteroids are minor planets, especially of the inner Solar System. Larger asteroids have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not resemble a planet-like disc and was not observed to have characteristics of an active comet such as a tail. As minor planets in the outer Solar System were discovered they were typically found to have volatile-rich surfaces similar to comets. As a result, they were often distinguished from objects found in the main asteroid belt. In this article, the term "asteroid" refers to the minor planets of the inner Solar System including those co-orbital with Jupiter.

With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets, along with thousands of significantly smaller bodies (asteroids and comets). For almost 50 years Pluto was thought to be larger than Mercury, [14] [15] but with the discovery in 1978 of Pluto's moon Charon, it became possible to measure Pluto's mass accurately and to determine that it was much smaller than initial estimates. [16] It was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was still more than ten times as massive as the largest object in the asteroid belt, Ceres, it had one-fifth the mass of Earth's Moon. [17] Furthermore, having some unusual characteristics, such as large orbital eccentricity and a high orbital inclination, it became evident that it was a different kind of body from any of the other planets. [18]

Comet Icy small Solar System body

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° across the sky. Comets have been observed and recorded since ancient times by many cultures.

Mercury (planet) Smallest and closest planet to the Sun in the Solar System

Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 87.97 days is the shortest of all the planets in the Solar System. It is named after the Roman deity Mercury, the messenger of the gods.

Charon (moon) largest natural satellite of the dwarf planet Pluto

Charon, also known as (134340) Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 km (377 mi). It was discovered in 1978 at the United States Naval Observatory in Washington, D.C., using photographic plates taken at the United States Naval Observatory Flagstaff Station (NOFS).

In the 1990s, astronomers began to find objects in the same region of space as Pluto (now known as the Kuiper belt), and some even farther away. [19] Many of these shared several of Pluto's key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects, plutinos. This led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the bodies now known as dwarf planets. [20] [21] By 2005, three trans-Neptunian objects comparable in size to Pluto (Quaoar, Sedna, and Eris) had been reported. [22] It became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. [23] Astronomers were also confident that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly if Pluto were to remain a planet. [24]

Eris (then known as 2003 UB313) was discovered in January 2005; [25] it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet . [26] As a consequence, the issue became a matter of intense debate during the IAU General Assembly in August 2006. [27] The IAU's initial draft proposal included Charon, Eris, and Ceres in the list of planets. After many astronomers objected to this proposal, an alternative was drawn up by the Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi: they proposed an intermediate category for objects large enough to be round but which had not cleared their orbits of planetesimals. Dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits. [28]

The IAU's final Resolution 5A preserved this three-category system for the celestial bodies orbiting the Sun. It reads:

Although concerns were raised about the classification of planets orbiting other stars, [29] the issue was not resolved; it was proposed instead to decide this only when such objects start to be observed. [28]


Euler diagram showing the types of bodies in the Solar System (except the Sun). Euler diagram of solar system bodies.svg
Euler diagram showing the types of bodies in the Solar System (except the Sun).

The term dwarf planet has itself been somewhat controversial, as it could imply that these bodies are planets, much as dwarf stars are stars. This is the conception of the Solar System that Stern promoted when he coined the phrase. The older word planetoid ("having the form of a planet") has no such connotation, and is also used by astronomers for bodies that fit the IAU definition. [30] Brown states that planetoid is "a perfectly good word" that has been used for these bodies for years, and that the use of the term dwarf planet for a non-planet is "dumb", but that it was motivated by an attempt by the IAU division III plenary session to reinstate Pluto as a planet in a second resolution. [31] Indeed, the draft of Resolution 5A had called these median bodies planetoids, [32] [33] but the plenary session voted unanimously to change the name to dwarf planet. [1] The second resolution, 5B, defined dwarf planets as a subtype of planet, as Stern had originally intended, distinguished from the other eight that were to be called "classical planets". Under this arrangement, the twelve planets of the rejected proposal were to be preserved in a distinction between eight classical planets and four dwarf planets. Resolution 5B was defeated in the same session that 5A was passed. [31] Because of the semantic inconsistency of a dwarf planet not being a planet due to the failure of Resolution 5B, alternative terms such as nanoplanet and subplanet were discussed, but there was no consensus among the CSBN to change it. [34]

In most languages equivalent terms have been created by translating dwarf planet more-or-less literally: French planète naine , Spanish planeta enano, German Zwergplanet , Russian karlikovaya planeta ( карликовая планета), Arabic kaukab qazm (كوكب قزم ), Chinese ǎixíngxīng ( 行星), Korean waesohangseong (왜소행성; 矮小行星), but in Japanese they are called junwakusei (準惑星) meaning "subplanets" or "almost-planets".

IAU Resolution 6a of 2006 [35] recognizes Pluto as "the prototype of a new category of trans-Neptunian objects". The name and precise nature of this category were not specified but left for the IAU to establish at a later date; in the debate leading up to the resolution, the members of the category were variously referred to as plutons and plutonian objects but neither name was carried forward, perhaps due to objections from geologists that this would create confusion with their pluton . [1]

On June 11, 2008, the IAU Executive Committee announced a name, plutoid , and a definition: all trans-Neptunian dwarf planets are plutoids. [11] The authority of that initial announcement has not been universally recognized:


Planetary discriminants [37]
BodyM/M (1)Λ (2)µ (3)Π (4)
Mercury 0.0551.95×1039.1×1041.3×102
Venus 0.8151.66×1051.35×1069.5×102
Earth 11.53×1051.7×1068.1×102
Mars 0.1079.42×1021.8×1055.4×101
Ceres 0.000158.32×10−40.334.0×10−2
Jupiter 317.71.30×1096.25×1054.0×104
Saturn 95.24.68×1071.9×1056.1×103
Uranus 14.53.85×1052.9×1044.2×102
Neptune 17.12.73×1052.4×1043.0×102
Pluto 0.00222.95×10−30.0772.8×10−2
Eris 0.00282.13×10−30.102.0×10−2
Sedna 0.000223.64×10−7<0.07 [38] 1.6×10−4

Showing the planets and the largest known sub-planetary objects (purple) covering the orbital zones containing likely dwarf planets. All known possible dwarf planets have smaller discriminants than those shown for that zone.

(1)Mass in M, the unit of mass equal to that of Earth (5.97 × 1024 kg).
(2)Λ is the capacity to clear the neighbourhood (greater than 1 for planets) by Stern and Levison. Λ = kM2a−3/2, where k = 0.0043 for units of Yg and AU, and a is the body's semi-major axis. [39]
(3)µ is Soter's planetary discriminant (greater than 100 for planets). µ = M/m, where M is the mass of the body, and m is the aggregate mass of all the other bodies that share its orbital zone.
(4)Π is the capacity to clear the neighbourhood (greater than 1 for planets) by Margot. Π = kMa−9/8, where k = 807 for units of Earth masses and AU. [40]

Orbital dominance

Alan Stern and Harold F. Levison introduced a parameter Λ (lambda), expressing the likelihood of an encounter resulting in a given deflection of orbit. [39] The value of this parameter in Stern's model is proportional to the square of the mass and inversely proportional to the period. This value can be used to estimate the capacity of a body to clear the neighbourhood of its orbit, where Λ > 1 will eventually clear it. A gap of five orders of magnitude in Λ was found between the smallest terrestrial planets and the largest asteroids and Kuiper belt objects. [37]

Using this parameter, Steven Soter and other astronomers argued for a distinction between planets and dwarf planets based on the inability of the latter to "clear the neighbourhood around their orbits": planets are able to remove smaller bodies near their orbits by collision, capture, or gravitational disturbance (or establish orbital resonances that prevent collisions), whereas dwarf planets lack the mass to do so. [39] Soter went on to propose a parameter he called the planetary discriminant, designated with the symbol µ (mu), that represents an experimental measure of the actual degree of cleanliness of the orbital zone (where µ is calculated by dividing the mass of the candidate body by the total mass of the other objects that share its orbital zone), where µ > 100 is deemed to be cleared. [37]

Jean-Luc Margot refined Stern and Levison's concept to produce a similar parameter Π (Pi). [40] It is based on theory, avoiding the empirical data used by Λ. Π > 1 indicates a planet, and there is again a gap of several orders of magnitude between planets and dwarf planets.

There are several other schemes that try to differentiate between planets and dwarf planets, [6] but the 2006 definition uses this concept. [1]

Hydrostatic equilibrium

Sufficient internal pressure, caused by the body's gravitation, will turn a body plastic, and sufficient plasticity will allow high elevations to sink and hollows to fill in, a process known as gravitational relaxation. Bodies smaller than a few kilometers are dominated by non-gravitational forces and tend to have an irregular shape. Larger objects, where gravitation is significant but not dominant, are "potato" shaped; the more massive the body is, the higher its internal pressure and the more rounded its shape, until the pressure is sufficient to overcome its internal compressive strength and it achieves hydrostatic equilibrium. At this point a body is as round as it is possible to be, given its rotation and tidal effects, and is an ellipsoid in shape. This is the defining limit of a dwarf planet. [41]

When an object is in hydrostatic equilibrium, a global layer of liquid covering its surface would form a liquid surface of the same shape as the body, apart from small-scale surface features such as craters and fissures. If the body does not rotate, it will be a sphere, but the faster it rotates, the more oblate or even scalene it becomes. If such a rotating body were to be heated until it melted, its overall shape would not change when liquid. The extreme example of a non-spherical body in hydrostatic equilibrium is Haumea, which is twice as long along its major axis as it is at the poles. If the body has a massive nearby companion, then tidal forces come into effect as well, distorting it into a prolate spheroid. An example of this is Jupiter's moon Io, which is the most volcanically active body in the Solar System due to effects of tidal heating. Tidal forces also cause a body's rotation to gradually become tidally locked, such that it always presents the same face to its companion. An extreme example of this is the Pluto–Charon system, where both bodies are tidally locked to each other. Earth's Moon is also tidally locked, as are many satellites of the gas giants.

The masses of the IAU-recognized dwarf planets plus Charon relative to the Moon. The mass of Makemake is a rough estimate. (See plutoid for a graph of several additional likely dwarf planets without Ceres.) Dwarf planet masses relative to Moon.svg
The masses of the IAU-recognized dwarf planets plus Charon relative to the Moon. The mass of Makemake is a rough estimate. (See plutoid for a graph of several additional likely dwarf planets without Ceres.)

The upper and lower size and mass limits of dwarf planets have not been specified by the IAU. There is no defined upper limit, and an object larger or more massive than Mercury that has not "cleared the neighbourhood around its orbit" would be classified as a dwarf planet. [42] The lower limit is determined by the requirements of achieving a hydrostatic equilibrium shape, but the size or mass at which an object attains this shape depends on its composition and thermal history. The original draft of the 2006 IAU resolution redefined hydrostatic equilibrium shape as applying "to objects with mass above 5×1020 kg and diameter greater than 800 km", [29] but this was not retained in the final draft. [1]

Empirical observations suggest that the lower limit will vary according to the composition and thermal history of the object. For a body made of rigid silicates, such as the stony asteroids, the transition to hydrostatic equilibrium should occur at a diameter of approximately 600 km and a mass of 3.4×1020 kg. For a body made of less rigid water ice, the limit should be about 320 km and 1019 kg. [43] In the asteroid belt, Ceres is the only body that clearly surpasses the silicaceous limit (though it is actually a rocky–icy body), and its shape is an equilibrium spheroid. 2 Pallas and 4 Vesta are rocky and are just below the limit. Pallas, at 525–560 km and 1.85–2.4×1020 kg, is "nearly round" but still somewhat irregular. Vesta, at 530 km and 2.6×1020 kg, deviates from an ellipsoid shape primarily due to a large impact basin at its pole.

Dwarf planets and possible dwarf planets

Illustration of the relative sizes, albedos, and colours of the largest trans-Neptunian objects TheTransneptunians Size Albedo Color.svg
Illustration of the relative sizes, albedos, and colours of the largest trans-Neptunian objects
Eight dwarves or Candidates and moon's orbits, shown at angle Eight TNO moonorbits.png
Eight dwarves or Candidates and moon's orbits, shown at angle

Many trans-Neptunian objects (TNOs) are thought to have icy cores and therefore would require a diameter of perhaps 400 km (250 mi)—only about 3% of that of Earth—to relax into gravitational equilibrium. [44] As of January 2015, about 150 known TNOs are considered potential dwarf planets, although only rough estimates of the diameters of most of these objects are available. [45] A team is investigating thirty of these, and think that the number will eventually prove to be around 200 in the Kuiper belt, with thousands more beyond. [44]


The IAU has recognized five bodies as dwarf planets since 2008: Ceres, Pluto, Eris, Haumea, and Makemake. [46] Ceres and Pluto are known to be dwarf planets through direct observation. [47] Eris is recognized as a dwarf planet because it is more massive than Pluto (measurements by New Horizons indicate that Pluto's diameter is larger than that of Eris), whereas Haumea and Makemake qualify based on their absolute magnitudes. [9] [35] In relative distance from the Sun, the five are:

  1. Ceres Ceres symbol.svg – discovered on January 1, 1801, 45 years before Neptune. Considered a planet for half a century before reclassification as an asteroid. Accepted as a dwarf planet by the IAU on September 13, 2006.
  2. Pluto ♇ – discovered on February 18, 1930. Classified as a planet for 76 years. Reclassified as a dwarf planet by the IAU on August 24, 2006.
  3. Haumea – discovered on December 28, 2004. Accepted by the IAU as a dwarf planet on September 17, 2008.
  4. Makemake – discovered on March 31, 2005. Accepted by the IAU as a dwarf planet on July 11, 2008.
  5. Eris – discovered on January 5, 2005. Called the "tenth planet" in media reports. Accepted by the IAU as a dwarf planet on September 13, 2006.


Liberal estimates are that another hundred or so known objects in the Solar System may be dwarf planets. [45] Such estimates are that up to 200 dwarf planets will be identified when the entire region known as the Kuiper belt is explored, and that the number may exceed 10,000 when objects scattered outside the Kuiper belt are considered. [48] Individual astronomers recognize several of these, [45] and Brown maintains a list of hundreds of candidate objects, ranging from "nearly certain" to "possible" dwarf planets. [49] However, it has been proposed that dark, low-density bodies smaller than 900–1000 km in diameter may retain internal porosity from their formation, never having fully collapsed into solid planetary bodies, [50] in which case they could not be dwarf-planets.

As of Feb 2018, Brown's list includes 952 objects, identifying ten known trans-Neptunian objects—the four accepted by the IAU plus the six bodies: 2007 OR10 , Quaoar, Sedna, Orcus, (307261) 2002 MS4 and Salacia—as "near certain", with another 16 "highly likely". [51] Notably, 2007 OR10 (1230 km) has a larger diameter than Pluto's largest moon Charon (1212 km).

Tancredi et al. advised the IAU to officially accept Orcus, Sedna and Quaoar. In addition, Tancredi considers the five TNOs Varuna, Ixion, 2003 AZ84 , 2004 GV9 , and 2002 AW197 to be dwarf planets as well. [47] Stern states that there are more than a dozen known dwarf planets. [48]

However, Grundy et al. cast doubt on the possibility that Salacia, Varda and similar objects are planetary bodies, while accepting that Orcus and Quaoar probably are (or at least are solid bodies). [50]

Objects recognized by the IAU as dwarf planets
Orbital attributes [52]
NameRegion of the
Solar System
radius (AU)
Orbital period
Mean orbital
speed (km/s)
to ecliptic
Ceres Asteroid belt 2.774.6017.88210.59°0.0790.33
Pluto Kuiper belt (plutino)39.48248.094.66617.14°0.2490.077
Haumea Kuiper belt (12:7)43.13283.284.53128.22°0.1950.020
Makemake Kuiper belt (cubewano)45.79309.94.41928.96°0.1590.02
Eris Scattered disc 67.675573.43644.19°0.4420.10
Physical attributes
relative to
the Moon
relative to
the Moon
(×1021 kg)


Moons Surface
Atmosphere H
Ceres 27%9461.3%0.942.170.290.51≈ 3°0.380167none3.3
Pluto 68%2380±0.417.8%13.051.870.581.2119.59°−6.39 5 44transient−0.76
Haumea ≈ 36%1632 [12]
5.5%4.01±0.041.885–1.757 [12]
2.6–3.3 (?)
0.440.840.16 2 32±3?0.2
Makemake 41%1430±14??> 1.4 [54] ?0.321≈ 30none [55] −0.3
Eris 67%2326±1222.7%16.72.5≈ 0.81.3≈ 1 (0.75–1.4) 1 ≈ 42transient?−1.1
Additional objects recognized by Brown and Tancredi as dwarf planets
(Salacia and 2002 MS4 are considered unlikely by Grundy et al.)
Orbital attributes [52]
NameRegion of the
Solar System
radius (AU)
Orbital period
Mean orbital
speed (km/s)
to ecliptic
Orcus Kuiper belt (plutino)39.17245.1820.57°0.2270.003
2002 MS4 Kuiper belt (cubewano)41.931271.5317.693°0.14135
Salacia Kuiper belt (cubewano)42.1889274.0323.9396°0.10312
Quaoar Kuiper belt (cubewano)43.405285.978.00°0.0390.007–0.010
2007 OR10 Scattered disc (10:3)67.21550.9830.70°0.500
Sedna Detached 518.57≈ 11,40011.93°0.853< 0.07
Physical attributes
relative to
the Moon
relative to
the Moon
(×1021 kg)


Moons Surface
Atmosphere H
Orcus 26%917±250.9%0.630.55 1 < 442.2
2002 MS4 ≈ 27%934±47??0≈ 433.7
Salacia ≈ 25%854±450.6%0.451.16+0.59
Quaoar 32%1110±51.8–2.0%1.4±0.10.74 1 ≈ 432.4
2007 OR10 ≈ 44%1230±502.4%1.751.721.8711.8
Sedna ≈ 30%995±80≈ 1.4%≈ 10.420≈ 121.5

Vesta, the next-most-massive body in the asteroid belt after Ceres, is roughly spherical, deviating mainly because of massive impacts that formed Rheasilvia and Veneneia crater after it solidified. [56] Furthermore, its triaxial dimensions are not consistent with hydrostatic equilibrium. [57] [58] Triton is thought to be a captured dwarf planet. [59] Phoebe is a captured body that, like Vesta, is no longer in hydrostatic equilibrium, but is thought to have been so early in its history. [60]


The dwarf planet Ceres, as imaged by NASA's Dawn spacecraft. PIA19562-Ceres-DwarfPlanet-Dawn-RC3-image19-20150506.jpg
The dwarf planet Ceres, as imaged by NASA's Dawn spacecraft.

On March 6, 2015, the Dawn spacecraft began to orbit Ceres, becoming the first spacecraft to orbit a dwarf planet. [61] On July 14, 2015, the New Horizons space probe flew by Pluto and its five moons. Dawn has also explored the former dwarf planet Vesta. Phoebe has been explored by Cassini (most recently) and Voyager 2, which also explored Neptune’s moon Triton. These three are thought to be former dwarf planets and therefore their exploration helps in the study of the evolution of dwarf planets.


In the immediate aftermath of the IAU definition of dwarf planet, some scientists expressed their disagreement with the IAU resolution. [6] Campaigns included car bumper stickers and T-shirts. [62] Mike Brown (the discoverer of Eris) agrees with the reduction of the number of planets to eight. [63]

NASA has announced that it will use the new guidelines established by the IAU. [64] Alan Stern, the director of NASA's mission to Pluto, rejects the current IAU definition of planet, both in terms of defining dwarf planets as something other than a type of planet, and in using orbital characteristics (rather than intrinsic characteristics) of objects to define them as dwarf planets. [65] Thus, in 2011, he still referred to Pluto as a planet, [66] and accepted other dwarf planets such as Ceres and Eris, as well as the larger moons, as additional planets. [67] Several years before the IAU definition, he used orbital characteristics to separate "überplanets" (the dominant eight) from "unterplanets" (the dwarf planets), considering both types "planets". [39]

Planetary-mass moons

Nineteen moons are known to be massive enough to have relaxed into a rounded shape under their own gravity, though some have since frozen out of equilibrium, and seven of them are more massive than either Eris or Pluto. These moons are not physically distinct from the dwarf planets, but do not fit the IAU definition of dwarf planet because they do not directly orbit the Sun. However, Alan Stern calls planetary-mass moons "satellite planets", one of three categories of planet, together with dwarf planets and classical planets. [67] The term planemo ("planetary-mass object") also covers all three populations. [68]

The seven moons that are more massive than Eris are Earth's Moon, the four Galilean moons of Jupiter (Io, Europa, Ganymede and Callisto), one moon of Saturn (Titan), and one moon of Neptune (Triton). Another moon of Saturn (Rhea), and likely one or two of Uranus (Titania and Oberon), are also in hydrostatic equilibrium. However, the other five ellipsoidal moons of Saturn (Mimas, Enceladus, Tethys, Dione and Iapetus) were once in equilibrium but are no longer. For the three other ellipsoidal moons of Uranus (Miranda, Ariel and Umbriel) and Pluto's moon Charon, the situation is unclear. There are additional possibilities of equilibrium moons among TNOs, including Eris's moon Dysnomia, though these are smaller and thus less likely.

In a draft resolution for the IAU definition of planet, both Pluto and Charon would have been considered dwarf planets in a binary system, given that they both satisfied the mass and shape requirements for dwarf planets and revolved around a common center of mass located between the two bodies (rather than within one of the bodies). [note 1] [29] The IAU currently states that Charon is not considered to be a dwarf planet and is just a satellite of Pluto, although the idea that Charon might qualify to be a dwarf planet in its own right may be considered at a later date. [69] The location of the barycenter depends not only on the relative masses of the bodies, but also on the distance between them; the barycenter of the Sun–Jupiter orbit, for example, lies outside the Sun.

See also


  1. The footnote in the original text reads: For two or more objects comprising a multiple object system.... A secondary object satisfying these conditions i.e. that of mass, shape is also designated a planet if the system barycentre resides outside the primary. Secondary objects not satisfying these criteria are "satellites". Under this definition, Pluto's companion Charon is a planet, making Pluto–Charon a double planet.

Related Research Articles

Double planet An informal term used to describe a planet with its moon

In astronomy, a double planet is a binary system where both objects are of planetary mass. The term is not recognized by the International Astronomical Union (IAU) and is therefore not an official classification. At its 2006 General Assembly, the International Astronomical Union considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned in favor of the current definition of planet. In promotional materials advertising the SMART-1 mission and pre-dating the IAU planet definition, the European Space Agency once referred to the Earth–Moon system as a double planet.

Solar System Planetary system of the Sun

The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.

90377 Sedna a large minor planet in the outer reaches of the Solar System

90377 Sedna, or simply Sedna, is a large minor planet in the outer reaches of the Solar System that was, as of 2015, at a distance of about 86 astronomical units (1.29×1010 km; 8.0×109 mi) from the Sun, about three times as far as Neptune. Spectroscopy has revealed that Sedna's surface composition is similar to those of some other trans-Neptunian objects, being largely a mixture of water, methane, and nitrogen ices with tholins. Its surface is one of the reddest among Solar System objects. It is a possible dwarf planet. Among the eight largest trans-Neptunian objects, Sedna is the only one not known to have a moon.

Michael E. Brown American planetary astronomer

Michael E. Brown is an American astronomer, who has been professor of planetary astronomy at the California Institute of Technology (Caltech) since 2003. His team has discovered many trans-Neptunian objects (TNOs), notably the dwarf planet Eris, which was originally thought to be bigger than Pluto.

Definition of <i>planet</i> definition of word planet

The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai, "wandering stars", for star-like objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids.

Dysnomia (moon) moon of the dwarf planet Eris

Dysnomia (Greek: Δυσνομία)—officially (136199) Eris I Dysnomia—is the only known moon of the dwarf planet Eris (the most massive known dwarf planet in the Solar System). It was discovered in 2005 by Mike Brown and the laser guide star adaptive optics team at the W. M. Keck Observatory, and carried the provisional designation of S/2005 (2003 UB313) 1 until officially named Dysnomia (from the Ancient Greek word Δυσνομία meaning anarchy/lawlessness) after the daughter of the Greek goddess Eris.

Eris (dwarf planet) dwarf planet in the Solar System

Eris is the most massive and second-largest dwarf planet known in the Solar System. Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its discovery was verified later that year. In September 2006 it was named after Eris, the Greek goddess of strife and discord. Eris is the ninth most massive object directly orbiting the Sun, and the 16th most massive overall, because seven moons are more massive than all known dwarf planets. It is also the largest which has not yet been visited by a spacecraft. Eris was measured to be 2,326 ± 12 kilometers (1,445.3 ± 7.5 mi) in diameter. Eris's mass is about 0.27% of the Earth mass, about 27% more than dwarf planet Pluto, although Pluto is slightly larger by volume.

Small Solar System body object in the Solar System that is neither a planet, nor a dwarf planet, nor a satellite

A small Solar System body (SSSB) is an object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union (IAU) as follows: "All other objects, except satellites, orbiting the Sun shall be referred to collectively as 'Small Solar System Bodies' ".

Plutoid trans-Neptunian dwarf planet

A plutoid or ice dwarf is a trans-Neptunian dwarf planet, i.e. a body orbiting beyond Neptune that is massive enough to be rounded in shape. The term plutoid was adopted by the International Astronomical Union (IAU) working group Committee on Small Bodies Nomenclature, but was rejected by the IAU working group Planetary System Nomenclature. The term plutoid is not widely used by astronomers, though ice dwarf is not uncommon.

50000 Quaoar Cold classical Kuiper belt object

50000 Quaoar, provisional designation 2002 LM60, is a non-resonant trans-Neptunian object (cubewano) and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It measures approximately 1,100 km (680 mi) in diameter, which is about half the diameter of Pluto. The object was discovered by American astronomers Chad Trujillo and Michael Brown at the Palomar Observatory on 6 June 2002. Signs of water ice on the surface of Quaoar have been found, which suggests that cryovolcanism may be occurring on Quaoar. A small amount of methane is present on its surface, which can only be retained by the largest Kuiper belt objects. In February 2007, Weywot, a synchronous minor-planet moon in orbit around Quaoar, was discovered by Brown. Weywot is measured to be 80 km (50 mi) across. Both objects were named after mythological figures from the Native American Tongva people in Southern California. Quaoar is the Tongva creator deity and Weywot is his son.

Planetary mass is a measure of the mass of a planet-like object. Within the Solar System, planets are usually measured in the astronomical system of units, where the unit of mass is the solar mass (M), the mass of the Sun. In the study of extrasolar planets, the unit of measure is typically the mass of Jupiter (MJ) for large gas giant planets, and the mass of Earth (M) for smaller rocky terrestrial planets.

Gonzalo Tancredi is an Uruguayan astronomer and full professor in the Department of Astronomy at the University of the Republic in Montevideo, Uruguay. He is an active member of the International Astronomical Union (IAU) and investigator at Los Molinos Observatory.


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